1. Field of the Invention
The present invention relates to a semiconductor device and to a method for fabricating the same. More particularly, it relates to a semiconductor device having a SiC layer and to a method for fabricating the same.
2. Description of the Related Art
A silicon carbide (SiC) is a semiconductor having a high breakdown field because of its band gap larger than that of silicon (Si) and stable even at a high temperature. For this reason, applications of SiC to power devices, RF devices, and high-temperature operating devices in the next generation have been expected. Crystals of SiC fall into a plurality of types including cubic 3C-SiC, hexagonal 6H-SiC and 4H-SiC, and rhombohedral 15R-SiC. Of the foregoing types, 6H-SiC and 4H-SiC have been used commonly to fabricate a practical SiC semiconductor device. On the other hand, a substrate having, as a principal surface, a surface substantially coincident with the (0001) plane perpendicular to the crystal axis has been used widely.
FIG. 11 is a cross-sectional view of a conventional vertical apparatus for growing a SiC thin film. As shown in the drawing, the conventional vertical apparatus for growing a SiC thin film is composed of: a reactor 100; a gas supply system 107; a gas exhaust system 111; and a coil 103 for heating a sample. A doping gas 106 is supplied to an upper inner portion of the reactor 100 from the gas supply system 107 through a pipe. A raw material gas 104 and a diluent gas 105 transported from the gas supply system 107 through different pipes are mixed with each other and supplied to the upper inner portion of the reactor 100. The pipes for supplying the raw material gas 104, the diluent gas 105, and the doping gas 106 to the reactor 100 are provided with respective flow meters 108, 109, and 110 for controlling flow rates. The reactor 100 is internally provided with a support shaft 114 extending vertically from the lower surface of the reactor 100 and a susceptor 101 supported by the support shaft 114. The raw material gas 104, the diluent gas 105, and the doping gas 106 are introduced into the reactor 100 and then exhausted therefrom via the gas exhaust system 111, as indicated by an arrow 112. The pressure in the reactor 100 is controlled via a valve 113. The susceptor 101 supported by the support shaft 114 is heated by RF induction heating using a coil 103 wound around the reactor 100. A cooling water is circulated in the peripheral portion 115 of the reactor 100.
A SiC crystal layer has been formed conventionally by a CVD method using the foregoing apparatus for growing a thin film. The following is the procedure for forming the SiC crystal layer.
First, a SiC substrate 102 is placed on the susceptor 101 in the reactor 100. Then, a hydrogen gas is introduced into the reactor 100 from the upper portion thereof and the pressure in the reactor is adjusted to a value equal to or lower than an atmospheric pressure. Under the condition, an RF power is applied to the coil 103 to heat the substrate 102 such that the substrate temperature is maintained at 1500xc2x0 C. or more.
Subsequently, a carbon containing gas (such as propane) and a silicon containing gas (such as silane) are introduced to allow the growth of a SiC crystal on the surface of the substrate 102.
In the case of forming a doped layer, a doping gas 106 (which is, e.g., nitrogen if an n-type doped layer is to be formed or aluminum if a p-type doped layer is to be formed) is supplied from the gas supply system 107 into the reactor 100 from the upper portion thereof through the flow meter 110.
After the completion of the crystal growth, the supply of the raw material gas 104 is stopped and the application of the RF power to the coil 103 is stopped, whereby the heating is terminated and the substrate 102 is cooled. As a result, a SiC crystal layer is formed on the SiC substrate.
FIG. 12 shows a process in which the respective flow rates of a raw material gas and a diluent gas and the temperature of a substrate vary with time in accordance with the conventional method for growing a SiC crystal layer. The supply of the diluent gas is initiated, while the substrate is heated. The raw material gas is supplied after the substrate temperature reaches a desired level.
In a SiC wafer, defects termed xe2x80x9cmicropipesxe2x80x9d which extend through the substrate exist. Therefore, it cannot be said that the whole wafer has an excellent crystalline property. Although studies have been made to reduce the micropipe defects in the SiC wafer, it is difficult with current technologies to prevent the occurrence of the micropipe defects in the SiC wafer.
When a semiconductor device is fabricated by using the wafer as a substrate with the conventional technologies, the micropipe defects in the substrate extend disadvantageously into the SiC thin film grown on the substrate by thermal chemical vapor deposition (CVD), thereby degrading the crystalline property. Even if the device is fabricated by using SiC for a substrate, it has been difficult to provide characteristics which can be expected from the excellent physical properties of SiC, including high-temperature operability and a high breakdown voltage.
It is therefore an object of the present invention to provide means for preventing micropipe defects in a substrate from extending into a SiC film and thereby allow the fabrication of a semiconductor device properly using the high-temperature operability and high breakdown voltage of SiC.
A semiconductor device according to the present invention comprises: a SiC substrate; a suppression layer formed on the SiC substrate, the suppression layer including at least one high-concentration SiC layer containing an impurity at a high concentration and having the function of preventing extension of a micropipe; and an active region formed on the suppression layer.
In the arrangement, the suppression layer prevents the upward growth of micropipe defects contained in the SiC substrate so that the density of the micropipe defects in the active region is lowered. This improves the characteristics of the semiconductor device including a breakdown voltage and high-temperature operability.
The suppression layer further includes: at least one low-concentration SiC layer containing an impurity at a concentration lower than in the high-concentration SiC layer and the high-concentration SiC layer and the low-concentration SiC layer are formed in alternately stacked relation. The arrangement causes a sharp change in impurity concentration between the high-concentration SiC layer and the low-concentration SiC layer and more effectively prevents the growth of the micropipe.
The active region includes: at least one first SiC layer functioning as a carrier flow region; and at least one second SiC layer containing an impurity for carriers at a concentration higher than in the first SiC layer, smaller in thickness than the first SiC layer, and allowing the carriers to be spread out into the first SiC layer under a quantum effect and the first SiC layer and the second SiC layer are formed in alternately stacked relation. The arrangement increases the mobility of the carriers in the active region and further improves the performance of the semiconductor device.
A method for fabricating a semiconductor device according to the present invention is a method for fabricating a semiconductor device comprising a suppression layer provided on a SiC substrate to prevent extension of a micropipe and an active region provided on the suppression layer, the method comprising: at least one suppression-layer forming step performed in accordance with a CVD method of epitaxially growing a high-concentration SiC layer, while pulsatively introducing an impurity.
In accordance with the method, the suppression layer prevents the upward growth of micropipe defects contained in the SiC substrate so that the density of the micropipe defects in the active region is lowered. This improves the characteristics of the semiconductor device including a breakdown voltage and high-temperature operability.
The suppression-layer forming step includes the step of: epitaxially growing a low-concentration SiC layer containing an impurity at a concentration lower than in the high-concentration SiC layer without introducing the impurity and the suppression-layer forming step is performed by alternately repeating the step of forming the high-concentration SiC layer and the step of forming the low-concentration SiC layer at least once.
The method causes a sharp change in the impurity concentration of the suppression layer and allows the formation of the suppressing which effectively prevents the growth of the micropipe defects. The micropipe defects disturb the crystalline structure therearound and thereby degrades the performance of the semiconductor device or lowers a production yield in the process of fabricating the semiconductor device. By suppressing the growth of the micropipe defects, therefore, the method allows the fabrication of a semiconductor device properly using a high breakdown voltage and high-temperature operability inherent in SiC and increases the production yield of the semiconductor device.
In the method according to the present invention, the impurity is one selected from the group consisting of nitrogen (N), phosphorus (P), aluminum (Al), boron (B), and neon (Ne). This allows the selected one of the elements to enter the lattice of a SiC crystal or enter an interstitial space and thereby effectively suppresses the growth of the micropipe defects.